250 amp Bat Charger

I have the above battery charger, there is no voltage at the clamps, I have tracked the problem down to the rectifier. The transformer still hums and doesn't heat up so I am assuming its not shorted. Is there a way to test the rectifier? What can I use to replace the rectifier that can handle the amps? Any help would be great Thanks Wolffe

Thanks for the answers it is a Selenium rectifier stack, I believe this would be difficult to find one like that would handle the amperage. I have checked at a surplus store where I find a lot of what I need but nothing that big. The unit is at the shop so I will get pics when I can. It's a Solar 500, http://www.centurytool.net/141_067_Solar_500_Battery_Charger_s/47.htm They no longer sell the rectifier here. As the guy before me stated switching it to diodes would be fine, if the parts could be found. Wolffe

Anybody know what the I-V characteristic of a Selenium rectifier is compared to a Silicon or Schottky? I suspect that the Selenium has a sizable forward resistance which partially limits the peak current from the transformer.

Wolffe,
Can you describe the rectifier circuit at all? Is it regulated by SCR devices, or is the secondary just rectified by the diode stack? Is it three phase? Is there a choke filter on secondary? Has the transformer got any labels? Do you want to use it for battery charging?

Wolffe,
Can you describe the rectifier circuit at all? Is it regulated by SCR devices, or is the secondary just rectified by the diode stack? Is it three phase? Is there a choke filter on secondary? Has the transformer got any labels? Do you want to use it for battery charging?

Ciao, Tim

Click to expand...

Very simple no SRC, not three phase, unknown on the choke I will look when I am there. No labels on the transformer. Yes I will use it again as a battery charger, it was an expensive one at the time.

If the transformer and other parts are rated for a 250A output, and it is a simple single phase rectifier, then the diode configuration needs to be determined - is the secondary a single winding (ie. 4 diodes in a full bridge), or is it centre tapped (ie. 2 diodes). (selenium stacks may be a bit bewildering to work out the diode configuration)

You also haven't indicated the nominal voltage?

But I caution that you really should aim to use some form of output voltage regulation and also current limit. You can seriously damage lead acid batteries with too high a recharge current if the battery is heavily discharged - the current limit needed is typically 20% C10 rating. You also can seriously damage lead acid batteries with too high a voltage, and have the risk of an explosion if not using a max voltage regulation limit.

If the transformer and other parts are rated for a 250A output, and it is a simple single phase rectifier, then the diode configuration needs to be determined - is the secondary a single winding (ie. 4 diodes in a full bridge), or is it centre tapped (ie. 2 diodes). (selenium stacks may be a bit bewildering to work out the diode configuration)

You also haven't indicated the nominal voltage?

But I caution that you really should aim to use some form of output voltage regulation and also current limit. You can seriously damage lead acid batteries with too high a recharge current if the battery is heavily discharged - the current limit needed is typically 20% C10 rating. You also can seriously damage lead acid batteries with too high a voltage, and have the risk of an explosion if not using a max voltage regulation limit.

Ciao, Tim

Click to expand...

We have a few of these type chargers at work for forklift batteries. These packs weight tons. Using this thing on normal batteries will cause an accident that will get you in the local 6 o'clock news. If your lucky your friends will call you "scar-face".

Thanks for the answers it is a Selenium rectifier stack, I believe this would be difficult to find one like that would handle the amperage. I have checked at a surplus store where I find a lot of what I need but nothing that big. The unit is at the shop so I will get pics when I can. It's a Solar 500, http://www.centurytool.net/141_067_Solar_500_Battery_Charger_s/47.htm They no longer sell the rectifier here. As the guy before me stated switching it to diodes would be fine, if the parts could be found. Wolffe

Click to expand...

HI, Your link goes to:
1. The Part number 17 in the illustration is the old style “Selenium rectifier”.

2. The exact same part that I replaced Years ago it with factory substution “Part number 865-679-666 Rectifier DISCONTINUED No Longer Available. No Replacement”.. as noted at the bottom of the link page.

My charger max amperage for starting is 500 amps and uses four of the daisy wheel designed diode packs. The parts offered in the links are for 250 amp service. The plastic daisy wheel seems a very poor design, mine is presently dead and disassembled on the bench.

It appears that a diode in the CPT50130 to CPT50145 range could work, if you have a way of keeping the diode package cool. You will need a large heat sink, most preferably copper, with forced air cooling. With 250A flowing through it, the diode will be dissipating roughly 150 Watts of power. If you don't have a way to get rid of the heat, the diode will fail very quickly.

It appears that a diode in the CPT50130 to CPT50145 range could work, if you have a way of keeping the diode package cool. You will need a large heat sink, most preferably copper, with forced air cooling. With 250A flowing through it, the diode will be dissipating roughly 150 Watts of power. If you don't have a way to get rid of the heat, the diode will fail very quickly.

Click to expand...

The unit has a fan for forced air and will have some type of heat-sink for the diode package.

The boost start amperage for the charger is 500 amps. How many CPT50130 Schottky's and what wiring schematic would you suggest?

What if anything would be necessary to preserve battery charge when the charger stops BUT is still connected to the battery?

What is your typical environment like where you will be using this charger?

and will have some type of heat-sink for the diode package.

Click to expand...

Do you mean to say that it does not have one now, so that your options are open?

The hotter and more humid the environment, the greater the airflow/size of heat sink needs to be in order to keep the diodes from getting fried. You'll be better off with a large heatsink and a small fan rather than the other way around.

The boost start amperage for the charger is 500 amps. How many CPT50130 Schottky's and what wiring schematic would you suggest?

Click to expand...

If you use a number of them in parallel, you will spread the heat dissipation over a larger area, while decreasing the forward voltage of each diode.

You will need a very heavy-duty copper bus bar to carry that much current.

What if anything would be necessary to preserve battery charge when the charger stops BUT is still connected to the battery?

Click to expand...

The diodes should take care of that. There will be some leakage current, but it will be relatively minor. If you are planning on leaving the charger connected for an indefinite period of time, you might consider a very heavy-duty relay or contactor that engages when power is applied to the charger, and disengages after charging is complete.

The first set of selenium rectifiers were replaced years ago with a poorly designed 'daisy wheel' diode setup. Six button diodes were placed in a plastic daisy wheel housing, four of these housing were stacked on one bolt. The 'heat sink' / conductor between the diode daisy wheels is 20 gauge galvanized steel sheet. This daisy wheel setup has also failed. The manufacturer does not offer any repair parts for this specific charger.

The Transformer was attached at the top and bottom holes to the left in the illustration. The hole on the right is where the negative lead for the charger connects. The positive lead connects to the transformer.

FAN - Factory original lubricated and running well, CFM... no clue.
Heat sink - Factory heat sink- 20 ga galvanized steel sheet has left much physical and design room to work with. The previous diode package was secured by one 1/4-20 machine screw mounted atop the transformer bracket. A minor change will be a major improvement, perhaps an aluminium heat sink of some design.
OPERATING Environment- Pennsylvania, general charging at moderate amperages; engine starting boost to the 500 amps ... a few times per month.
Charging TIME- the charger has a timer that stops charging after the number of minutes it is set at. So when the timer is set for 120 minutes and left to charge overnight the battery is fully charged in the morning.

Sheet steel was a very poor choice for a conductor and heat sink. It has a relatively high thermal resistance and electrical resistance; several times that of aluminum.

While aluminum is much better than steel, copper is nearly twice as good as aluminum in both thermal transfer characteristics and electrical conductivity. It is also somewhat more resistant to corrosion. When aluminum oxidizes, it changes to a white powdery substance that is a very poor conductor of heat/electricity, which is why they don't use it for house wiring any longer.

From your description and the images, it appears that the transformer has a center-tapped secondary, and is wired like the 2nd schematic in this image (bottom left):

In such a scheme, each diode conducts for 1/2 cycle. If your average current is 500 Amperes, the load is split between D2A and D2B.

The diodes you are considering have a Vf of about 0.55V at 250A. Since power in Watts = voltage x current, that's roughly 138 Watts of power, but at a 50% duty cycle, ~69 Watts each.

The diodes also have a thermal limit (150°C) and a thermal coupling junction to mount rating of 0.06°C per Watt of power dissipation if thermal grease is used, which is quite good. At 69 Watts, if the heat sink were ideal, the diode would only rise 4.14°C above ambient (room temperature).

Let's look at bus bar material.

Here's a helpful site: http://stormcopper.com/wordpress/?p=490
After you read that page, look at the ampacity tables, here: http://www.stormcopper.com/design/ampacity.htm
If you scan through the tables, you'll see that even with a 1/8"x2" bus bar, you will get a 30°C temp rise at 495 Amperes. I would personally want to go thicker than that; and have more surface area as well. You need the thickness particularly at the electrical/thermal connection points, and you need the surface area to radiate the heat. Having a strong fan moving air across the surfaces will help a great deal to keep things cool.

You need to decide whether you wish for the fix to be permanent, or if you want to re-build it every couple of years. If you think copper is expensive now, it's a bargain compared to a couple years from now. You will be better served to err on the side of caution and overkill than to try to save a couple of bucks.

One very important thing that has not been discussed as of yet, is what is the no-load voltage that you measure on the transformer secondary, from end-to-end, and from the center tap to either end? Please let us know what you read using the (RMS) AC scale on your meter. If it exceeds 21 VAC from center to end, or 42VAC from end to end, you will have to use those diodes in series, as otherwise the breakdown voltage will be exceeded and the diodes destroyed.

Attached Files:

Here is a basic conceptual drawing (not to scale) for your connections and heat sink:

The orange areas are copper. I am only going to recommend copper, not only for the thermal and electrical conductivity issues, but also for the thermal coefficient of expansion. The diodes you are considering have a copper base plate. If mounted to copper bus bars, they will have a perfect match for the thermal coefficient of expansion, which means almost no stress when heated. Aluminum has a very high thermal coefficient of expansion, which would tend to tear the diodes in half.

Note carefully that because the diodes are a common cathode configuration, the cables must be swapped; the new configuration will be like the upper schematic in my prior post. The transformer center tap becomes the battery - (ground/negative/black) cable connection, and the lower rectifier plate becomes the battery + cable connection.

Only two diodes are shown. More could be added if desired, by extending the drawing up and down. Three or four would be good; four preferable. I don't know for certain how much space you have available. Four diodes would provide for a symmetrical configuration, and less power dissipation in the rectifier assembly itself. The lower the current passing through each diode, the lower the forward voltage, and the lower the power dissipation. Four in parallel would put each diode at 1/2 it's max current rating at 1/2 duty cycle, which is very good.

If the top two bus bars were 3-1/2 to 4" wide (each) and at least 3/16" thick pure copper, you'll probably be OK. I suggest spacing the diodes at least 2" apart. The bottom bus will need to be more than twice as wide as the top, obviously. If you use 4 diodes, then the assembly will be at least 10" long.

If you don't have good machine skills or access to a drill press, you might just have their team make up some bus bars for you - but it would probably be cheaper for you to go to a Big Blue or Big Orange store and buy a drill press.

Make certain that there are no ridges around the holes before you install the diodes. Countersinks will remove surface burrs. Sanding the buses using some wet-dry sandpaper laid a sheet of glass will get rid of the high spots. After that, it's thermal grease and elbow grease (tightening the bolts/screws properly)

If you are tempted to save a bit on the copper, you could double up the thickness right in the areas where the connections are. Sweat-solder some copper stock or copper washers on top and/or on the bottom. Make certain the copper is absolutely clean before soldering. 3M Scotchbrite(tm) pads with some 90% or better isopropyl alcohol work great. Carburetor cleaner or lacquer thinner also work, but are more hazardous than the isopropyl alcohol.

Note that the standoffs must be completely insulated. You might use nylon screws and nuts, and nylon, Teflon, or ceramic spacers.

There needs to be good air flow over the top, in between and underneath the bus bars/plates.

If you wish to increase the surface area, you could fabricate some fins by bending copper sheet into an "L" shape, and sweat-solder the bottom of the "L" to the tops of the upper and bottom of the lower plates/buses in line with the desired air flow direction (see note in blue on drawing). Note that the two upper plates must be electrically insulated from each other, or a direct short across the transformer secondary will result in lots of smoke.

The best possible arrangement for cooling would be to mount the rectifier assembly vertically, with the fan pulling in cool air from the bottom and blowing it out the top.

Maintenance will always be an issue, regardless of how well it is engineered and constructed. You should open it up every six months or so, and make certain that the fan blades are clean, and that no dust/dirt has accumulated on the rectifier assembly, and double-check the electrical connections to make certain they haven't worked themselves loose. Using thermal compound on the screws/nuts/bolts will help a good bit.

Corrosion-X is a really good product. It was developed to help fight corrosion in military aircraft aboard aircraft carriers. It's not cheap, but a couple blasts of spray Corrosion-X should keep those copper plates and connections bright and clean for the foreseeable future. I think a spray can costs about $10.

If you don't want to spend that much, you might just give it a light coat of high-temp flat black paint. Note that your finger oils are corrosive, and also will prevent paint from adhering wherever you have touched the surfaces. Isopropyl alcohol, 90% or better, will remove the finger oils - and it is one of the more benign cleaners. Be aware that isopropyl alcohol is flammable, and burns with a nearly invisible blue flame. Lacquer thinner works, but will strip part numbers right off.

Brake cleaner works too, but can be extremely hazardous to your health, so is ABSOLUTELY NOT recommended for use on ANYTHING electrical. Please read this article to discover the risks of brake cleaner: http://www.brewracingframes.com/id75.htm
There is nothing funny about phosgene gas.